The long-term goal of the proposed research is to develop a novel quantitative flow through cell counting method for enumeration of CD4+ T lymphocytes in human blood. CD4 cell counts are needed to stage and monitor HIV-infected patients. In adults, the absolute number of CD4+ T cells per microlitre of blood has critical prognostic and therapeutic implications, and is used for both HIV staging and treatment decisions. The gold standard technique currently used for quantitation of CD4 cells is flow cytometry. However, flow cytometers, including smaller single-purpose CD4 cell counting devices developed recently are expensive and inaccessible in resource-limited countries. In addition, they require trained laboratory technicians to handle and process samples and to analyze and report data. Because flow cytometry-based CD4 cell counting is beyond the reach of so many HIV-infected people, the World Health Organization (WHO) has urged the development of simple point-of-care CD4 cell count assays designed for resource-limited settings. It is in this context that the less expensive, rapid and easy-to- perform method, which is free from the limitations of the conventional assays, for determination of CD4+ T cells is proposed to be developed. The proposed method, flow-through cell counting assay (FTCA) is based on the use of the special membrane and a proprietary flow-through cassette to measure cell count status. The test can be completed in less than 15 minutes and can be performed by laboratory technicians with low levels of training. The specific aims of Phase I include designing and testing a proof-of-concept prototype of the FTCA cassette, examination of the performance of the FTCA using several commercially available membranes and various anti-CD4 antibodies conjugated with labels of different origin and optimization of concentrations of the reagents and assay conditions for performing of FTCA. Advantages of the proposed technology such as the speed, convenience, and low cost of the analysis substantiates our belief that the FTCA will be widely used in clinical practice. PUBLIC HEALTH RELEVANCE: The development of the proposed method will allow using CD4 cell counting tests for staging and monitoring HIV infection. Successful completion of these studies will be beneficial for public health, and make available all of the technology needed for a substantial business opportunity to license the technology and manufacture commercial products.